GIVING TREES WHAT THEY NEED

Karen L. Kerkhoff KKERKHOFF@PRIMEDIABUSINESS.COM

We invest a lot of time in the growth of our landscape trees. The time may not be measured as actual time spent directly cultivating the trees, but measured in how many years have passed since the trees were planted. And after 5, 10 or 20 years invested in encouraging the trees to grow, what do we have to show for it? Researchers at North Carolina State University summarized the standards soil must meet for adequate, even vigorous growth. They found soils that are 2- to 3-percent organic matter are ideal for tree growth. Soils with less organic matter don't support the needs of the trees. Thin soils, such as those located over bedrock or high water tables, are less productive than a soil profile approximately 30 inches deep. Conversely, the area of soil exposed to the atmosphere is critical. Given 100 cubic feet of soil, the tree growing in a 10-foot by 10-foot surface area performs better than the same volume with only 1-foot by 1-foot exposed. Even though the soil is deeper, there is not enough surface area for oxygen to percolate down into the root zone. Trees in heavily amended soils run the risk of falling over as the organic matter decays and is not replaced. Street trees installed in “potting soils” may need permanent staking. Street tree wells can become hopelessly compacted and contaminated with de-icing salts. Can this soil be replaced with minimal disturbance of the root system? Given good, competition-free soils and growing environment, tree roots may grow as much as 10 to 15 feet per year.